This invention relates to semiconductor memory systems, particularly to non-volatile memory systems, and have application to flash electrically-erasable and programmable read-only memories (EEPROMs).
Flash EEPROM systems are being applied to a number of applications, particularly when packaged in an enclosed card that is removably connected with a host system. Current commercial memory card formats include that of the Personal Computer Memory Card International Association (PCMCIA), CompactFlash (CF), MultiMediaCard (MMC) and Secure Digital (SD). One supplier of these cards is SanDisk Corporation, assignee of this application. Host systems with which such cards are used include personal computers, notebook computers, hand held computing devices, cameras, audio reproducing devices, and the like. Flash EEPROM systems are also utilized as bulk mass storage embedded in host systems.
Such non-volatile memory systems include an array of floating-gate memory cells and a system controller. The controller manages communication with the host system and operation of the memory cell array to store and retrieve user data. The memory cells are grouped together into blocks of cells, a block of cells being the smallest grouping of cells that are simultaneously erasable. Prior to writing data into one or more blocks of cells, those blocks of cells are erased. User data are typically transferred between the host and memory array in sectors. A sector of user data can be any amount that is convenient to handle, preferably less than the capacity of the memory block, often being equal to the standard disk drive sector size, 512 bytes. In one commercial architecture, the memory system block is sized to store one sector of user data plus overhead data, the overhead data including information such as an error correction code (ECC) for the user data stored in the block, a history of use of the block, defects and other physical information of the memory cell block. Various implementations of this type of non-volatile memory system are described in the following United States patents and pending applications assigned to SanDisk Corporation, each of which is incorporated herein in its entirety by this reference: U.S. Pat. Nos. 5,172,338, 5,602,987, 5,315,541, 5,200,959, 5,270,979, 5,428,621, 5,663,901, 5,532,962, 5,430,859 and 5,712,180, and application Ser. Nos. 08/910,947, filed Aug. 7, 1997, and 09/343,328, filed Jun. 30, 1999. Another type of non-volatile memory system utilizes a larger memory cell block size that stores multiple sectors of user data.
One architecture of the memory cell array conveniently forms a block from one or two rows of memory cells that are within a sub-array or other unit of cells and which share a common erase gate. U.S. Pat. Nos. 5,677,872 and 5,712,179 of SanDisk Corporation, which are incorporated herein in their entirety, give examples of this architecture. Although it is currently most common to store one bit of data in each floating gate cell by defining only two programmed threshold levels, the trend is to store more than one bit of data in each cell by establishing more than two floating-gate transistor threshold ranges. A memory system that stores two bits of data per floating gate (four threshold level ranges or states) is currently available, with three bits per cell (eight threshold level ranges or states) and four bits per cell (sixteen threshold level ranges) being contemplated for future systems. Of course, the number of memory cells required to store a sector of data goes down as the number of bits stored in each cell goes up. This trend, combined with a scaling of the array resulting from improvements in cell structure and general semiconductor processing, makes it practical to form a memory cell block in a segmented portion of a row of cells. The block structure can also be formed to enable selection of operation of each of the memory cells in two states (one data bit per cell) or in some multiple such as four states (two data bits per cell), as described in SanDisk Corporation U.S. Pat. No. 5,930,167, which is incorporated herein in its entirety by this reference.
Since the programming of data into floating-gate memory cells can take significant amounts of time, a large number of memory cells in a row are typically programmed at the same time. But increases in this parallelism causes increased power requirements and potential disturbances of charges of adjacent cells or interaction between them. U.S. Pat. No. 5,890,192 of SanDisk Corporation, which is incorporated herein in its entirety, describes a system that minimizes these effects by simultaneously programming multiple chunks of data into different blocks of cells located in different operational memory cell units (sub-arrays).
There are several different aspects of the present invention that provide improvements in solid state memory systems, including those described above. Each of these aspects of the present invention, the major ones being generally and briefly summarized in the following paragraphs, may be implemented individually or in various combinations.
Multiple user data sectors are programmed into a like number of memory blocks located in different units or sub-arrays of the memory array by alternately streaming data from one of the multiple sectors at a time into the array until a chunk of data is accumulated for each of multiple data sectors, after which the chunks are simultaneously and individually stored in respective blocks in different units of the memory. This increases the number of memory cells that may be programmed in parallel without adverse effects.
An error correction code (ECC), or other type of redundancy code, may be generated by the controller from the streaming user data during programming and written into the same memory block as the user data from which it is generated. The redundancy code is then evaluated by the controller when the sector of data is read out of the memory block. A single redundancy code generation circuit is utilized, even when the streaming data is alternated between data chunks of the multiple sectors, by providing a separate storage element for each of the user data sectors being programmed at the same time, in which intermediate results of the generation are temporarily stored for each sector.
Overhead data of the condition, characteristics, status, and the like, of the individual blocks are stored together in other blocks provided in the array for this purpose. Each overhead data record may include an indication of how many times the block has been programmed and erased, voltage levels to be used for programming and/or erasing the block, whether the block is defective or not, and, if so, an address of a substitute good block, and the like: A group of blocks are devoted to storing such records. A large number of such records are stored in each of these overhead blocks. When accessing a specific user data block to perform one or all of programming, reading or erasing, the overhead record for that user data block is first read and its information used in accessing the block. By storing a block""s overhead data outside of that block, frequent rewriting of the overhead data, each time the user data is rewritten into the block, is avoided. It also reduces the amount of time necessary to access and read the block overhead data when the block is being accessed to read or write user data. Further, only one ECC, or other redundancy code, need be generated for the large number of overhead records that are stored in this way.
The records from a number of overhead blocks can be read by the controller into an available portion of its random-access memory for ease of use, with those overhead blocks whose records have not been accessed for a time being replaced by more active overhead blocks in a cache-like manner. When a beginning address and number of sectors of data to be transferred is received by the memory controller from the host system, a logical address of the first memory block which is to be accessed is calculated in order to access the overhead record for that block but thereafter the overhead records are accessed in order without having to make a calculation of each of their addresses. This increases the speed of accessing a number of blocks.
Information of defects in the memory, such as those discovered during the manufacturing process, may also be stored in separate blocks devoted for this purpose and used by the controller so that the imperfect memory circuit chips may be included in the memory system rather than discarding them. This is particularly an advantage when a single defect record affects many blocks. One such defect is a bad column that is shared by a large number of blocks. A number of bad column pointers (BCPs) may be stored together as a table in one or more sectors that are devoted in part or entirely to this overhead data. When this is done, the physical location of the streaming user data being written to the memory is shifted when a comparison of the relative physical location within a sector of individual bytes of data with the BCP table indicates that the data byte is being directed to at least one memory cell that is along a bad column. The reverse is performed during reading, where data bits read from memory cells that were skipped over during write because of a bad column are ignored.
Since flash EEPROM cells have, by their nature, a limited life in terms of the number of times that they can be erased and reprogrammed, it is usually prudent to include one or more operational features that tend to even out the wear on the various memory blocks that can be caused by multiple rewrites to the same blocks. One such technique alters from time-to-time the correspondence between the digital data and the memory states that are designated to represent the digital data. To accomplish this in the present memory system, the first one or few bits of the initial byte of the individual sectors of data, termed herein as a flag byte, are used to designate such correspondence. These bits are designated upon writing user data, and all data following the initial bits are transformed on the fly as the streaming data is being transferred to the memory array in accordance with their value. Upon reading a sector of data, these initial bit(s) are read and used to transform back all subsequent data stored in the sector to their original values as the data are being read out of the memory in a stream.
When the memory system is formed of multiple memory cell arrays, such as by using two or more integrated circuit chips that each include such an array, the system""s manufacture and use is simplified by accumulating information about each of the memory arrays in the system, and then storing that information as a single record in some convenient location, such as in one of the blocks of one of the memory arrays. This makes it much easier to combine memory arrays having different sizes and/or operating characteristics into a single system. One such record merges the number of blocks of user data available in each of the memory chips in a way that establishes a continuum of logical block addresses of the blocks of all the arrays in the system. When a location of memory is being accessed for a read or write operation, the memory controller then accesses the merged record in a process of converting a logical block address to a physical address of a block in one of the memory arrays.
Such a merged record can be automatically generated and stored during manufacturing by the controller reading the information from each of the memory arrays, merging that information into a single record, and then writing that record into a designated block of one of the memory arrays. Currently, the memory controller is usually provided on a separate integrated circuit chip, with one or more memory cell array chips connected with the controller. It is contemplated, in light of continuing processing technology improvements, that a memory array can also be included on the controller chip. When that amount of memory is insufficient for a particular system, one or more additional circuit chips containing further memory array(s) are then utilized. When two or more physically separate arrays of included in a system, then the generation of the merged record simplifies operation of the controller to address blocks across multiple arrays.
Other characteristics of various memory array circuit chips used to form a system, such as optimal voltage ranges, timing, numbers of pulses used, characteristics of voltage pumps, locations of overhead blocks, and the like, can vary without adverse effects. These operating characteristics can also be tabulated into a single system file for access by the micro-controller, or, more conveniently, the micro-controller can be operated to first access those of such characteristics as are necessary from an individual memory array chip before accessing that chip to read or write data. In either case, this allows the memory system to be formed with memory chips having different characteristics without any degradation in its performance. The manufacturing of non-volatile memory systems is simplified since all the memory array chips in a system need not be the selected to be the same.
Additional aspects, features and advantages of the present invention are included in the following description of specific embodiments, which description should be taken in conjunction with the accompanying drawings.